Contents

Introduction

Solar telescopes are based on the same construction principles as the night-time instruments, but solar observations require specialized telescopes and instruments, since they must withstand the heat input from the Sun while maintaining their
high resolution in the spatial, spectral and temporal dimensions. The solar irradiation heats up the ground, causing a layer of turbulent air which in turn degrades the image quality. Solar telescopes are therefore normally installed in towers, above this turbulent layer.

Most of the existing solar telescopes are synoptic instruments with apertures ranging form a few centimeters to, say, half a meter. Several of these telescopes are organized as networks for helioseismology measurements. Others monitor solar activity and provide images of the solar disk at different wavelength bands, or magnetograms. These telescopes often provide important background information for high-resolution studies, although their importance has somewhat decreased, since the SOHO satellite delivers daily full-disk images without interruptions. New full disk telescopes offer synoptic data at a high cadence, for the investigation of short-lived phenomena.

Telescopes with apertures larger than, say, 0.5 meters have a field of view of only a small fraction of the solar disk at an image scale that allows for diffraction-limited imaging in the focal plane. In the past, most telescopes of the 0.5 to 1 meter class had evacuated light paths in order to suppress inhomogeneities of the air’s index of refraction caused inside the telescope by thermal input from the Sun. For solar telescopes of the next generation with apertures of 1.5 to 4 meters, open structures are envisaged, with complex cooling systems for the primary optics that avoid the heating due to absorbed solar radiation. Optical elements are made of material with very low thermal expansion and, if possible, with high thermal conductivity. The latter property simplifies the cooling process and significantly shortens the time needed to reach thermal equilibrium.

Many of the phenomena that can be observed in the solar atmosphere have a lifetime of only a few minutes, and important changes may occur within a few seconds. High-resolution solar telescopes therefore have to provide light levels high enough to achieve a sufficiently high signal-to noise level. This is most important for the measurement of the (weak) magnetic field in the solar photosphere. Important small-scale objects have sizes of 100 km or less, and it requires telescopes with an aperture of at least one meter to resolve them. Next-generation telescopes with apertures of about four meters will be able to achieve high light level, short integration time, and good spatial resolution. It should be noted that for diffraction-limited observations, the light level per resolution element is the same for any telescope. For an increased light level one therefore has to sacrifice spatial or temporal resolution.

In this article we mainly discuss properties of high-resolution telescopes and the corresponding instrumentation. We do not attempt to provide a full list of existing telescopes, but mention only very few ones that represent important construction principles, and that are scientifically very successful, thanks to their adaptive optics systems.

Best sites

High-quality solar observations require sites with low levels of local and high-altitude turbulence. The atmosphere should also contain little water vapor and dust particles, in order to minimize the amount of scattered light. Sites on high mountains located on rather small islands have proven to be the best solar sites. Low levels of local turbulence can also be obtained at lake sites, where the nearby water keeps the ambient air temperature fluctuations low and inhibits the build-up of local turbulence. The comprehensive ATST solar site site survey, arguably the most testing so far, identifies three excellent site for solar observations: Mees Solar Observatory on Hawaii, Observatorio del Roque de los Muchachos on La Palma and Big Bear Solar Observatory in California. There is evidence that Antarctic sites such as Concordia Station at Dome C might have also have excellent day-time seeing. In the future, the quality of solar telescope sites may be more precisely characterized by the number and altitude of turbulence layers in the atmosphere above the telescope. Multi-conjugated adaptive optics systems (see below) will be able to correct the image degradation caused by such well-defined layers.

Existing telescopes

A large number of solar telescopes with apertures between 150 cm and about 10 cm is presently operational around the globe (see e.g. Landoldt-Börnstein for a list of solar telescopes). Many of the small-aperture telescopes are either used for routine observations of the full disk (images of the chromosphere, magnetograms of the photosphere), or are organized in networks for helioseismic measurements. Three of the large-aperture telescopes are presently equipped with adaptive optics, and are therefore suited for observations with highest possible spatial resolution, for imaging and spectroscopy. The Dunn Solar Telescope (DST, Sunspot, NM, 1969), the German Vacuum Tower Telescope (VTT, Tenerife, 1987), and the Swedish 1-meter Solar Telescope (SST, La Palma, 2002) have a number of common features, but also important differences. All three telescopes (i) are tower constructions with the telescope entrance high above ground, above the local layer of turbulence, (ii) have a long focal length of the primary mirror or lens, to avoid a hot focal plane, (iii) use evacuated tubes for the light path, and (iv) are domeless or with retractable dome. The DST and the SST have a altitude-azimuth feed system (“Turret”) that allow to have the full light path in vacuum, whereas the VTT uses a Coelostat system. The SST is a refractor, with a 1-m lens that acts also as entrance window.

Next generation solar telescopes

At present, three solar telescopes of the 1.5 to 2 meter class are in preparation or under construction, and two of them should become operational within the next one or two years. These telescopes mark an important design change, since they do no longer rely on evacuated or helium-filled telescope tubes to avoid turbulent air in the light path. They represent an intermediate step between the presently available telescopes and the next-generation 4 meter telescopes.
The next generation of solar telescopes with apertures in the range of 4 meters have been enabled by two technical breakthroughs: adaptive optics for solar telescopes, and the feasibility of air-cooled, open telescopes. The Dutch Open Telescope (DOT) on La Palma has been a pathfinder for the new generation of open telescopes.

The German GREGOR telescope has an aperture of 1.50 m and is located at the Observatorio del Teide on Tenerife. It is an open telescope in a three-mirror Gregory configuration with a focal length of 50 m. The primary mirror is made of CESIC, a silicon-carbide material with high thermal conduction, and it is air-cooled from the backside.
At Big Bear Solar Observatory, the New Solar Telescope is under construction. It is an open off-axis Gregory system with an aperture of 160 cm and an effective focal length of 88 m. Both telescopes will be equipped with high-order adaptive optics and become operational before 2010. In India, a project to build a 2-meter telescope in the Himalaya, at an altitude of 5000 m, has been initiated.

In the US, the Advanced Technology Solar Telescope (ATST) project of the National Solar Observatory is ready to enter in the constructions phase. The construction phase is expected to start in 2009, and First Light may occur in 2014. The ATST is a 4-meter, off-axis telescope, and it will be constructed on the Haleakala mountain (3000 m) on Hawaii. The telescope design is optimized for high sensitivity, polarimetric accuracy and low scattered light. Due to its open design, the telescope covers a wavelength range from 0.3 µm to 35 µm.
The COronal Solar Magnetism Observatory (COSMO), a coronagraph with an aperture of 1.5 meters has been proposed by the High Altitude Observatory in Boulder, and the Universities of Hawaii and Michigan. Phase-A studies for this project are currently underway.
In 2007, the European Association for Solar Telescope (EAST) has initiated the European Solar Telescope (EST) project. EST is a telescope of the 4-meter class, to be built in the Canary Islands toward the end of the second decade. During a design study, carried out between 2008 and 2010, the opto-mechanical design of EST will be worked out, and a local site characterization will be made. EST will measure the solar magnetic field with highest spatial and spectral resolution in the visible and near infrared wavelength region.

Instrumentation

Adaptive optics

The recent development of real-time adaptive optics systems to measure and stabilize image motion and to compensate low- and high-order image aberrations led to a major breakthrough in the spatial resolution of solar observations. To date, several telescopes of the 70 – 100 cm class are equipped with adaptive optics systems. These systems can correct atmospheric disturbances with a bandwidth of up to 100 Hz and are capable of correcting the dominant aberration modes caused by the turbulent Earth atmosphere and the instrument itself. The number of aberration modes that can be corrected grows with the number of sub-apertures of the wave-front sensor. The typical size of sub-apertures of a high-order adaptive optics is about 8 cm. This is small enough to account for the anisoplanatism of the day-time atmosphere, but also large enough to resolve the solar photospheric granulation. The area that can be corrected with adaptive optics is very small, only a few arcseconds in diameter. In order to overcome this limitation, multi-conjugated adaptive optics systems are presently under development. Theses systems use several deformable mirrors to correct the wavefront deformations that occur in different heights above the telescope.

The importance and the complexity of adaptive optics for solar observations grows rapidly with increasing telescope aperture. The achievable spatial resolution of the planned telescopes in the U.S. and Europe with apertures in the order of 4 meters will depend critically on the quality of their adaptive optics systems. A high-order system will require wave-front sensors with about 2000 sub-apertures – quite a technical challenge. Fortunately, computing power has grown more rapidly than the size of telescopes, therefore such high-order systems are nowadays within reach.

Filtergraphs

Observations of the smallest details on the Sun, near the diffraction limit of the telescope are made with broad-band imagers. They may consist of a filter to select the wavelength band, and a suitable digital detector, e.g. a CCD camera. Thanks to the high light level, exposure times of a few milliseconds are sufficient. This allows collecting bursts of images in rapid sequence and then selecting the very best ones afterwards, or use the full burst for post-facto image restoration using techniques based on multi-frame blind deconvolution or speckle interferometry. These techniques allow the study of the morphology of the solar photosphere and the evolution of large- and small-scale objects on time scales of a seconds, minutes or longer. Without image restoration, the high-quality field of view of filtergrams is limited by the corrected field (isoplanatic area) of the adaptive optics. However, in practice the full field of view of the available CCDs can be restored to homogeneous quality.

Spectroscopic instruments

Spectroscopic instruments are needed to obtain physical parameters, such as temperature, magnetic field, or flow speed. These measurements are multi-dimensional: two spatial dimensions, wavelength, and time. At present, detectors can only record two dimensions at a time. There are two different solutions to obtain spectroscopic data: filter instruments that record two-dimensional images at a fixed wavelength, and long-slit spectrographs that record one spatial dimension and a certain wavelength range. Both types of instruments have obvious advantages and also disadvantages, and it depends on the scientific topic, which one is preferred. Some solar observatories therefore provide both instruments.

Filter spectrometers record (nearly) monochromatic images. They use tunable narrow-band filters to select the wavelength. Spatial and wavelength information is recorded by taking a sequence of monochromatic images with varying wavelength. Tunable filters can be Lyot filters, or Fabry-Pérot Interferometers or Michelson Interferometers. With a combination of two or three tunable high-quality Fabry-Pérots, a spectral resolution of 2.5 pm can be obtained. The global tuning range is around 300 nm. Due to the small free spectral range, the spectral coverage for an individual measurement is limited to 0.3 nm.
Filter spectrometers are often equipped with an additional broad-band channel that takes images in a fixed wavelength band, and simultaneous with the narrowband images. The broad-band sequences are then used for post-facto reconstruction of the data. A typical data set with a field of view of about one arc minute squared and 15 wavelength positions across a spectral line can be taken in a few seconds. The spatial resolution of such a measurement depends on the size of the telescope and the image scale on the detector. Different parts of the spectral line are measured at different times. During times of variable seeing, the shape of the line profile may become distorted. Several Fabry-Pérot instruments are available at the high-resolution solar telescopes mentioned above.

Long-slit grating spectrographs provide instantaneous information about a certain wavelength range and one spatial dimension (along the slit). The spectral resolution depends mainly on the (illuminated) area of the diffraction grating and the focal length of the instrument. Compact spectrographs have a resolution of 2.5 pm (Resolving power of 250.000), similar to the best filter spectrometers. Spectrographs with large gratings and long focal lengths, like the Echelle spectrograph of the German VTT, have a theoretical resolving power of 1.000.000. Slit spectrographs record one or several spectral lines at a time. This is important for the investigation of the shape of line profiles, because they are not distorted by possible changes in the Earth atmosphere. Two-dimensional spatial information is collected by moving the solar image across the slit. The time needed to cover a certain area depends on the desired spatial resolution, i.e., the slit width and the step size. Fast cadences with high spectral resolution and coverage are possible for small scan areas. Grating spectrographs cover a large range of wavelengths, typically from 380 to 2200 Nanometers.

Spectro-polarimeters are used for the measurement of magnetic field in the solar atmosphere. They exist as combination of filter spectrometers or long-slit spectrographs with suitable polarization modulation components. Since the fraction of polarized light from the Sun is often very small, the needed accuracy of polarimetric measurements is very high. The magnetic signal is obtained by measuring the Stokes parameters that provide information about the total intensity, the circular and two orthogonal states of linear polarization. The polarization modulation is performed either with rotating retarding wave plates, or with modern tunable liquid crystal retarders.
A single magnetic field measurement requires at least four different images at different settings of the polarization modulator. In order to minimize the influence of variable seeing conditions, these images have to be taken in rapid sequence. In addition, precise calibrations of the polarization properties of the telescope and the spectro-polarimeter itself are necessary, to guarantee high polarimetric accuracy of the data. The best instruments have an accuracy of 1 part in 10,000.